Power Quality's Most Wanted

Even without the pictures on the post office wall or the television show of almost the same name, being able to recognize the most popular power quality “criminals” can help quickly find the source of problems. Though we don’t have room to post the top 10, below you’ll find the top three, representative of common types of problems.

Sags

When the voltage varies between 10 percent and 90 percent of the nominal value for 1/2 cycle to 3 minutes, then it is called a sag. Reductions in voltage from the nominal value are the most common type of PQ phenomena in just about every survey taken. Looking at the timeplot of the RMS voltage and current and/or the associated waveforms can let you know where the sag originated, what caused it, and even ways to prevent it. The most recognizable sag has the rectangular or bathtub shaped rms timeplot. The plot goes straight down and then horizontal for typically 3-10 cycles before going straight back up to nominal. This sag is usually the result of a high current fault on the utility distribution system, which will last until the overcurrent protection circuit decides to open the protection breaker at the substation.

Within a facility, a common sag picture comes from the start of medium to large horsepower motors. This “classic profile” is seen in Figure 1. When a motor starts up, there is usually a large inrush current of 6 to 10 times the steady state current levels, lasting from milliseconds to seconds, depending on the size of the motor and the load on it. The gradual exponential decay of the current after the initial inrush results in a voltage sag that begins abruptly, but then gets less severe over time. Like the distribution circuit fault that resulted in a sag, the large current levels result in a larger voltage drop in the source impedance, leaving less generated voltage for the loads.

Transients

Though most people buy TVSS outlet strips as the solution to PQ problems (remember they do nothing for sags or harmonics), they can be effective for positive impulsive transients that exceed the clamping voltage of the surge suppressors. However, a more common type of transient recorded on monitoring instruments is the power factor correction capacitor. This is classified as an oscillatory transient, and is recognized by: the negative transient when the uncharged capacitor is first energized; followed by a positive transient resulting from the response of the inductor-based impedance of the wiring system; and finally, the oscillations between 400 and 2000 Hz, based on the resonant point of the inductance, capacitance and resistance of the circuit. This transient can be severe enough to trip adjustable speed drives off line, as the overcurrent circuitry reacts to the incoming voltage waveform.

Harmonics

Single phase electronic loads, such as personal computers, laser printers and fax machines, draw harmonic currents of the third, fifth, seventh, ninth, 11th harmonic and so on, in decreasing amplitudes. That would lead many people to think that the third harmonic should be the most common harmonic found in electrical systems. That may be true within some facilities, but since the larger amount of current drawn is in the industrial realm, the recent surveys have found the fifth harmonic to be the most dominant. This is because the “front end” of adjustable speed drives and other equipment usually have three-phase, full-wave rectifiers that turn AC into DC. This is a six-pulse or pole converter (3 x 2), which results in harmonic patterns that follow the Hn = N x P +/- 1. A 6-pulse converter has harmonic currents of 5 and 7 (6 x 1 + 1 and 6 x 1 - 1), followed by 11 and 13 (6 x 2 +/- 1), 17 and 19, and so on. This makes the fifth harmonic the most dominant harmonic on distribution systems, as found in the EPRI DPQ project.

Other sources

There are a number of PQ books on the market that have a good selection of “signature” or “most wanted” waveforms in them, along with explanations of what caused them, and in some cases, how to make them go away and become that non-notable, but often desired, pure sine wave. Whereas most equipment will usually run on non-perfect voltage waveforms, the better looking the PQ mug shot, the less likely that you are to have PQ problems. EC

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.